Semiconductor image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals. A back side illuminated (BSI) image sensor device is one type of image sensor devices. These BSI image sensor devices are operable to detect light from its backside.
The conventional sensor, called the front side illumination (FSI) image sensor for these CMOS chips, is constructed in a fashion similar to the human eye, and has a lens at the front, layers of metal having wiring in the middle, and photo detectors on a silicon substrate (which absorbs the light) at the back. These metal layers may not only deflect the light on the sensor, they could also reflect it, reducing the incoming light captured by the photo detectors. By contrast, the back side illuminated (BSI) sensor has the same elements as FSI, but orients the wiring behind the photo detectors layer by flipping the silicon wafer during manufacturing and then thinning its reverse side so that light will hit the silicon first, and the photo detectors layer without passing through the wiring layer. This change can improve the chance of an input photon being captured from about 60% to over 90%, and the sensitivity per unit area to deliver better low-light shots.
A BSI image sensor device typically has a radiation-absorption region or a radiation-absorption region, a periphery region, and a bonding pad region. The radiation-absorption region has a silicon substrate that includes an array or grid of pixels formed inside for sensing and recording an intensity of electromagnetic radiation or wave (such as light) entering the substrate from the backside, and some circuitry and input/outputs adjacent the grid of pixels for providing an operation environment for the pixels and for supporting external communication with the pixels. After the grid of pixels and the circuitry and input/outputs are formed within the substrate, the substrate is thinned from its backside to a desired thickness, the backside of the substrate in the radiation-absorption region is covered by one or more anti-reflective (AR) layers and a sacrificial dielectric layer or film. To enhance the absorption of radiation by the substrate, it is important to reduce the thickness of the sacrificial dielectric layer to an optimum value so that the resulting dielectric film may, together with the AR layers, effectively reduce the reflection of radiation at the surface of the substrate.
The conventional method in the current art uses a plasma etching process to remove part of the sacrificial dielectric and reduce it to a thin dielectric film of a desired thickness. The conventional method, however, has many problems. First, with the plasma etching process, it is very difficult to control the thickness of the film to obtain a desirable degree of uniformity and profile within the wafer or even within the die. Further, the capability of the etching process of obtaining uniformity in film thickness is also easily influenced by prior processes of forming the oxide layer that may have been impaired or deviated. Second, the plasma etching process will damage the dielectric film surface to cause a surface roughness problem.
Therefore, for enhancing the efficiency of radiation absorption and uniformity for higher quality and performance of a backside illuminated (BSI) image sensor device, it is desirable to provide a method of precisely controlling the oxide film thickness formed on the radiation-absorption region of the image sensor device, which the conventional etching process cannot do.